1. Technical Field
This invention relates to depositing a material into a selected shape then modifying the initial material to provide desirable mechanical properties. Inclusion of small amounts of an additive allows selective heat treatment to give a material with improved material properties when compared to the properties of the deposited material. In particular, a shaped, soft material may be coated with the new material and heat treated to give a shaped, hardened coating, particularly in the form of a conductive spring.
2. Description of Related Art
The concept of applying a coating to impart desirable mechanical properties is used in many fields, from semiconductors to automobiles. For example, micro-electromechanical structures, microelectronic packaging, and magnetic storage media all employ such coatings. A variety of processes from sputtering to electroforming to chemical vapor deposition are widely used to fabricate such coatings. However, the mechanical properties of many of these coatings are not completely stable, especially at elevated temperatures. This is especially true for deposition processes that result in a non-equilibrium structure. Thus parts with these coatings have a fundamental problem in applications which require stable mechanical properties under load, particularly at elevated temperatures.
Annealing, or heating a material at a significantly elevated temperature for an extended period of time, is generally recognized as a way to bring a structure more into equilibrium. Annealing is often used to relieve brittleness. Brittleness often results from various material forming processes. For example, in forming a wire, it is common to extrude the material through a die, which involves various compression and deformation processes. The wire as extruded has the desired shape, but examination of the microstructure of the material reveals large amounts of internal stresses as internal stress fields. If these internal stress fields are high, the material might be considered to be brittle, and will break under moderate applied stresses. Heat treating such a wire will allow the material to reorganize and relieve these internal stress fields.
Heat treatment also is used to redistribute components within a system. For example, it is common in semiconductor processing to apply a dopant such as boron or phosphorous on the surface of a silicon substrate. Heating, or annealing, this product allows redistribution of the dopant atoms within the silicon structure as the dopant atoms diffuse within the base material.
Annealing of metal coatings such as nickel (Ni) also is common in many plating operations. It is quite common to electroplate nickel on a substrate, then anneal at, for example, 700.degree. C. for one to two hours. This is generally to relieve essentially all stress in the coating, so the annealing is continued for a relatively long time and/or at a relatively high temperature. In traditional applications, nickel is plated relatively quickly, which gives a relatively disordered initial structure, which in turn provides many sources for residual stress fields. Annealing allows the material to reach an equilibrium structure, which is much more stable.
Note that a typical annealing heat treatment involves both time and temperature and one skilled in the art can balance higher temperatures against shorter times or vice versa.
Another traditional process is the preparation of thin films for hard disk or other recording surfaces. A thin film of material such as NiP is deposited on a substrate, then annealed to give a hard material.
As semiconductor technology advances and the density of devices on chips increases, increased demands are placed on electrical interconnections in microelectronics packaging and microelectronics diagnostics. The mechanical properties of such interconnections are important in achieving reliable packaging and diagnostic solutions.
For example, it is typically desirable for such interconnections to have some resiliency. Currently, commonly-used technologies in microelectronics packaging exhibit little or no resiliency. Typical packaging includes wirebonding, tape automated bonding (TAB), solder bump technology, pin-in-hole solder, pin brazing, and surface-mount solder. While "pogo" pins used in microelectronics diagnostics are designed to have a resilient mechanical structure, their substantial inductance inhibits the use of high frequency signals by the diagnostics system.
Other resilient structures useful in microelectronics include a class of structures known as micro-electronic mechanical structures or MEMS. A number of researchers have fabricated small structures such as horizontal beams positioned with other electronic components to make devices such as relays. A variety of gears and other mechanical structures have been prepared.
Before the present invention, there was perceived a need to form strong, resilient microstructures but there was no technology that would allow this. Forming microstructures directly from a resilient material is in general quite difficult, if not impossible, in that a resilient material resists specific shaping methods. For example, tungsten needle in convention probe cards can be bent at a 90.degree. angle, positioned, then cut to length, but subtler shaping is extremely difficult.
Before the present invention, it was not possible to plate a coating on a substrate of small (tens to hundreds of microns) or even large (millimeters, centimeters, or larger) minimum feature size to provide resilient characteristics, particularly where it was desirable to have a structure with good mechanical yield strength. This limitation was particularly troublesome when the device was intended for use at moderately elevated temperatures, temperatures in excess of 100.degree. C., 85.degree. or even 50.degree. C. Conventional coatings could not be used to create durable, strong spring structures due to the thermal instability of the resulting coated products. The lack of structures with useful mechanical properties made it extremely difficult to build devices with large numbers of small springs, devices such as a probe card.
Early work in formable microstructures showed that a soft material such as gold could be shaped readily, then plated to give a hard coating and a resilient structure. See U.S. Patent No. 5,476,211, issued Dec. 19, 1995, assigned to FormFactor, Inc, entitled "Method of Manufacturing Electrical Contacts, Using a Sacrificial Member." The work that lead to the present invention showed that use of improved materials, and subsequent heat treatment could provide a strong, resilent final product. The use of these same or similar materials together with appropriate heat treatment can provide a resilient structure in a wide variety of applications.
Plating techniques in general are well known. See, for example, U.S. Pat. No. 4,439,284, "Composition Control of Electrodeposited Nickel-Cobalt Alloys." However, the selection of plating materials and the heat treatment conditions disclosed herein have not been disclosed in the past, in the '284 patent or elsewhere.
One skilled in the art will recognize other applications in which a material with high yield strength would be beneficial. This is particularly true for base materials with an arbitrary and possibly complex shape where retention of that shape is important, or where the base material does not have a sufficiently high yield strength. In particular, when making various spring structures of equivalent geometry and scale, an increase in elastic modulus will increase the spring value proportionally. For a fine-pitch interconnect, achieving greater spring value in a fixed volume is beneficial.